This invention generally relates to a method of hydrocarbon well completion and the associated apparatus for practicing the method. More particularly, the invention provides an open hole gravel packing system wherein a positive hydrostatic pressure differential within the well borehole is maintained against the production formation walls throughout all phases of the gravel packing procedure.
To extract hydrocarbons such as natural gas and crude oil from the earth""s subsurface formations, boreholes are drilled into hydrocarbon bearing production zones. To maintain the productivity of a borehole and control the flow of hydrocarbon fluids from the borehole, numerous prior art devices and systems have been employed to prevent the natural forces from collapsing the borehole and obstructing or terminating fluid flow therefrom. One such prior art system provides a full depth casement of the wellbore whereby the wellbore wall is lined with a steel casing pipe that is secured to the bore wall by an annulus of concrete between the outside surface of the casing pipe and the wellbore wall. The steel casing pipe and surrounding concrete annulus is thereafter perforated by ballistic or pyrotechnic devices along the production zone to allow the desired hydrocarbon fluids to flow from the producing formation into the casing pipe interior. Usually, the casing interior is sealed above and below the producing zone whereby a smaller diameter production pipe penetrates the upper seal to provide the hydrocarbon fluids a smooth and clean flowing conduit to the surface.
Another prior art well completion system protects the well borewall production integrity by a tightly packed deposit of aggregate comprising sand, gravel or both between the raw borewall and the production pipe thereby avoiding the time and expense of setting a steel casing from the surface to the production zone which may be many thousands of feet below the surface. The gravel packing is inherently permeable to the desired hydrocarbon fluid and provides structural reinforcement to the bore wall against an interior collapse or flow degradation. Such well completion systems are called xe2x80x9copen holexe2x80x9d completions. The apparatus and process by which a packed deposit of gravel is placed between the borehole wall and the production pipe is encompassed within the definition of an xe2x80x9copen hole gravel pack system.xe2x80x9d Unfortunately, prior art open hole gravel pack systems for placing and packing gravel along a hydrocarbon production zone have been attended by a considerable risk of precipating a borehole wall collapse due to fluctuations in the borehole pressure along the production zone. These pressure fluctuations are generated by surface manipulations of the downhole tools that are in direct fluid circulation within the well and completion string.
Open hole well completions usually include one or more screens between the packed gravel annulus and a hydrocarbon production pipe. The term xe2x80x9cscreenxe2x80x9d as used herein may also include slotted or perforated pipe. If the production zone is not at the bottom terminus of the well, the wellbore is closed by a packer at the distal or bottom end of the production zone to provide bottom end support for the gravel pack volume. The upper end of the production zone volume is delineated by a packer around the annulus between the wellbore and the pipe column, called a xe2x80x9ccompletion stringxe2x80x9d, that carries the hydrocarbon production to the surface. This upper end packer may also be positioned between the completion string and the inside surface of the well casing at a point substantially above the screens and production zone.
Placement of these packers and other xe2x80x9cdownholexe2x80x9d well conditioning equipment employs a surface controlled column of pipe that is often characterized as a xe2x80x9ctool stringxe2x80x9d. With respect to placement of a gravel pack, a surface controlled mechanism is incorporated within the tool string that selectively directs a fluidized slurry flow of sand and/or gravel from within the internal pipe bore of the tool string into the lower annulus between the raw wall of the wellbore and the outer perimeter of the completion string. This mechanism is positioned along the well depth proximate of the upper packer. As the mechanism directs descending slurry flow from the tool string bore into the wellbore annulus, it simultaneously directs the rising flow of slurry filtrate that has passed through screens in a production pipe extended below the upper packer. This rising flow of slurry filtrate is directed from the production pipe bore into the wellbore annulus above the upper packer.
It is during the interval of manually manipulated change in the slurry flow direction that potential exists for creating a hydrostatic pressure environment within the wellbore annulus below the upper packer that is less than the natural hydrostatic pressure of fluid within the formation. Such a pressure imbalance, even briefly, may collapse the borehole or otherwise damage the productivity of the production zone borehole wall or damage the filter cake. Highly deviated or horizontal production zone boreholes are particularly susceptible to damage due to such a pressure imbalance. Consequently, it is an object of the present invention to provide a flow cross-over mechanism that will provide a positive (overburden) pressure against a borehole wall throughout all phases of the gravel packing process.
It is also an object of the invention to provide a procedure and mechanism for maintaining fluid pressure on the production zone wellbore wall below the upper packer that is at least equal or greater than the natural hydrostatic pressure after the packer is set and while a greater fluid pressure is imposed on the wellbore annulus above the upper packer for testing the seal integrity of the packer.
Another object of the present invention to provide an apparatus design that facilitates a substantially uniform overburden pressure within a borehole production zone throughout the cross-flow changes occurring during a gravel packing procedure.
A preferred embodiment of the present invention includes a gravel pack extension tube that is permanently secured within a wellbore casing; preferably in or near the well production zone thereof. Near the upper end of the gravel pack extension tube is a packing seal that obstructs fluid flow through an annular section of the casing between the internal casing wall and the external perimeter of the gravel pack extension tube. The lower end of the gravel pack extension tube includes an open bore pipe that may be extended below the casing bottom and along the open borehole into the production zone. The distal end of the lower end pipe is preferably closed with a bull plug. Along the lower end of the pipe extension, within the hydrocarbon production zone and above the bull plug, are one or more gravel screens that are sized to pass the formation fluids while excluding the formation debris.
Internally, the upper end of the gravel pack extension tube provides two, axially separated, circular seal surfaces having an annular space therebetween. Further along the gravel pack extension tube length, several, three for example, axially separated, axial indexing lugs are provided to project into the extension tube bore space as operator indicators.
The dynamic or operative element of the present packing apparatus is a crossover flow tool that is attached to the lower end of a tool string. Concentric axial flow channels around the inner bore channel are formed in the upper end of the upper end of the crossover flow tool. An axial indexing collet is secured to the crossover tool assembly in the axial proximity of the indexing lugs respective to the extension tube. A ball check valve rectifies the direction of fluid flow along the inner bore of the crossover flow tool. A plurality of transverse fluid flow ports penetrate through the outer tube wall into the concentric flow channels. Axial positionment of the crossover flow tool relative to the inner seals on the gravel pack extension seals controls the direction of fluid flow within the concentrically outer flow channels. At all times and states of flow direction within the gravel packing procedure and interval, the production zone bore wall is subjected to at least the fluid pressure head standing in the wellbore above the production zone by means of the transverse flow channels and the concentric outer flow channels.